Abstract

Small scale distribution and diurnal migration of zooplankton were investigated in lake Naardermeer, a shallow lake largely covered by uniform Chara beds. For sampling, pattern samplers with a number of inverted funnels facing towards the lake bottom and held in a frame were used. Samplers were placed near the water surface and just above the plants, during one diel cycle (noon and midnight). Ceriodaphnia and Bosmina showed the highest levels of aggregation, both in the Chara bed and in the water above it. Chydorus sphaericus Müller and Diaphanosoma brachyurum Leivendid not aggregate in open water at daytime. Chydorus sphaericus and Eurytemora velox Lilljeborg ascended from the Chara bed into the open water during the night. Sida crystalina Müller and Chydorus sphaericus individuals collected at night were larger in size than those collected during daytime, both in the Chara bed and in the water above it. The results indicate that larger Sida crystallina individuals have a stonger preference to stay in the Chara bed at night than smaller individuals. In lakes with extensive and uniform Chara beds with a large overlying water column, zooplankton appear to migrate vertically rather than horizontally [KEYWORDS: Chara lake, diurnal vertical migrations, pattern sampler, chydorids, Sida crystallina, Bosmina]